Crosstalk identification in xDSL systems

ABSTRACT

Methods, apparatus and systems for identifying crosstalk interference in xDSL systems are disclosed and are useful in a variety of xDSL systems to assist in the provisioning, maintenance and diagnosis of the xDSL system and in spectral management and assignments. Signal data are collected from a receiver, a primary transmitter and any crosstalk transmitters. The signal data are resampled, if necessary. A first estimate of the timing offset between the received signal and each crosstalk signal is then obtained by cross-correlating the received data with the transmitted crosstalk data. The first timing offset estimate is then used in connection with a least-squares estimation of the crosstalk response for the considered crosstalk data and a second estimate of the timing offset. The invention may be used at a third party site remote from the system transmitters and receivers. The crosstalk identification of the present invention can be used in dynamic spectrum management for DSL services and signals.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional No.__/___,___, (Attorney Docket No. STFUP017P) filed on Feb. 6, 2001, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to data transmission systems and,more particularly, to the identification of crosstalk interference in acommunication system.

[0003] Digital subscriber line (DSL) technology uses the existingtelephone twisted pairs to provide high-speed internet access servicesto both residential and business customers. There are many types ofDSLs, which are generically referred to as xDSL, including basic rateDSL (ISDN), high-bit-rate DSL (HDSL), second generation HDSL (HDSL2),asymmetric DSL (ADSL), symmetrical DSL (SDSL), and very-high-bit-rateDSL (VDSL). Today in the United States, several million telephone linesbetween central offices and subscribers are deployed with xDSLtechnology, and the number of the subscribers is rising rapidly.

[0004] These wide band modulation approaches present inherent obstaclesthat must be overcome. One particular problem relates to crosstalkinterference that is introduced to the twisted pair transmission lineand received by the modem. As is well known to those skilled in the art,crosstalk interference is unwanted interference (signal noise) that ispassed between adjacent network cables or devices. Crosstalk generallyoccurs due to coupling between wire pairs when wire pairs in the same ora nearby bundle are used for separate signal transmission. In thismanner, data signals from one or more sources may be superimposed on andcontaminate a data signal from a second source. The crosstalk includesnear-end crosstalk (NEXT) and far-end crosstalk (FEXT). In ADSL and VDSLsystems, frequency-division duplexing can be used to avoid NEXT.Nevertheless, NEXT may still exist because of other types of serviceslike ISDN, HDSL, HDSL2, SDSL and T1.

[0005] As can be appreciated, the data signals being transmitted overthe twisted-pair phone lines can be significantly degraded by thecrosstalk interference generated on one or more adjacent twisted-pairphone lines in the same and/or a nearby bundle. As the speed of the datatransmission increases, the problem worsens. For example, in the case ofVDSL signals being transmitted over the twisted-pair phone lines, thecrosstalk interference can cause significant degradation of the VDSLsignals, including substantially limiting the maximum data rate of anindividual line. To prevent a breakdown of currently deployed systems,operators frequently assume and compensate for the worst case scenario(that is, the highest level of crosstalk interference). However, thisassumption is often too pessimistic when compared to actual crosstalkinterference on the transmission line and hence unnecessarily limits theoverall performance of the system. If actual crosstalk interferencecould be identified, then the crosstalk could either be removed (orlessened) or the system could be operated in a manner that does notunnecessarily compensate for a level of crosstalk that is not present.

[0006] Identification of crosstalk coupling functions within telephonelines can yield several overwhelming benefits. First, the crosstalkfunctions can be used in a multi-user detector in a line's modem tocancel the strong interference from other lines. Second, it can improvethe data rate (or the line reach scope) of a system by better spectrummanagement, such as a better spectrum assignment for different users.For example, if one user causes strong crosstalk to another user in aparticular frequency band, the modem may be switched to avoidtransmitting in this frequency band in lieu of a multi-user detector.Third, crosstalk profiles are invaluable for the telephone operators tomaintain, diagnose, and expand the current systems.

[0007] However, it has proven to be exceedingly difficult to identifycrosstalk functions among copper wires because lines in the same bundlecould belong to different service operators as a result of theunbundling process and regulatory action undertaken in many parts of theworld. For example, in the United States and some other countries,competitive local exchange carriers (CLECs) can lease the telephonelines from incumbent local exchange carriers (ILECs, the traditionalphone companies) and offer xDSL services to the local subscribers. As aresult, the modems from different operators are asynchronous. Evenwithin the same service operator, different types of services (HDSL,ADSL, ISDN, etc.) are offered in the same bundle and these services havedifferent symbol rates.

[0008] Because the modems in the same bundle could belong to differentservice operators (CLECs and ILECs), the time stamps of the data fromdifferent operators' modems can be offset by several milliseconds.Therefore, an additional problem with identifying crosstalk interferencewith existing xDSL systems is the presence of timing differences betweenthe transmitted data from different users and the received data from onedesignated receiver. Currently, the timing difference between twosignals can be greater than one thousand data symbols.

[0009] Moreover, in the multi-operator environment, spectralcompatibility among the different operators is a major concern. Spectralcompatibility is fundamentally determined by the crosstalk level causedby different users. For the foregoing reasons, some level ofcoordination and agreement in which all operators' interests are fairlyconsidered and benefited would be helpful to all users of such xDSLsystems.

[0010] Therefore, crosstalk problems arising from using twisted-pairphone lines with high data Transmission rates, including ADSL and VDSLfor example, become a substantial impediment to a receiver being able toproperly receive the transmitted data signals. Thus, there is a need toprovide techniques to identify and determine the timing differences invarious data signals and to identify and determine the magnitude andphase of crosstalk interference so that steps can be taken to reduce oreliminate such interference, improve line maintenance and assist inspectrum assignment.

SUMMARY OF THE INVENTION

[0011] Broadly speaking, the present invention is a technique foridentifying crosstalk interference in xDSL systems. The technique isuseful in a variety of xDSL systems and can be used to assist in theprovisioning, maintenance and diagnosis of the xDSL system and inspectral management and assignments. The invention can be implemented innumerous ways, including as a method, system or modular crosstalkidentifier.

[0012] In one embodiment, the invention relates to a method ofidentifying crosstalk interference in a received data signal. Initially,signal data are collected from a receiver, a primary transmitter and anycrosstalk transmitters. The signal data are resampled, if necessary. Afirst estimate of the timing offset between the received signal and eachcrosstalk signal is then obtained by cross-correlating the received datawith the transmitted crosstalk data. The first timing offset estimate isthen used in connection with a least-squares estimation of the crosstalkresponse for the considered crosstalk data and a second estimate of thetiming offset.

[0013] In some embodiments, multiple crosstalk signals are superimposedon the primary signal. If the crosstalk signals are all of comparablestrength, they can be identified together. If some of the crosstalksignals are materially stronger than others, the relatively strongcrosstalk signals are identified and removed from the received signal.Thereafter, the method can be re-applied to weaker signals to identifyand remove crosstalk interference in successive applications of themethod.

[0014] In other embodiments, the method of the present invention can beapplied as part of a multiuser detection service, provisioning of xDSLservices, xDSL diagnosis services, xDSL system maintenance servicesand/or spectral management within an xDSL system.

[0015] The invention relates, in another embodiment, to a system foridentifying crosstalk in which a number of transmitters transmit varioussignals, including a primary transmitter that transmits a primary signalto a receiver. A processor includes a data collector that collects datafrom the transmitters and the receiver. A timing offset estimator in theprocessor takes the collected data and generates a first estimate of atiming difference between the primary signal and each transmittedsignal. Each first estimate of the timing offset corresponding to atransmitted signal is then used by a crosstalk identifier in theprocessor to calculate the crosstalk response function corresponding tothat transmitted signal.

[0016] In other embodiments, the timing offset estimator uses across-correlator to perform a cross-correlation of the primary signaland each of the other transmitted signals. A least-squares estimator isused by the crosstalk identifier thereafter to identify the crosstalkresponse. The processor can be located anywhere in the system, but maybe located at a site remote from the receiver and the transmitters.

[0017] Another embodiment of the present invention is a crosstalkidentifier having a collector configured to collect data from a primarysignal transmitter, at least one crosstalk transmitter and a receiver.The crosstalk identifier contains a first estimator that determines afirst estimate of the timing offset between the data collected from thereceiver and the data collected from the crosstalk transmitters. Thisfirst estimate of the timing offset is used by a second estimator in thecrosstalk identifier to determine the crosstalk response function of thecrosstalk transmitter's data. A cross-correlator can be used in thefirst estimator and a least-squares estimator can be used in the secondestimator. The crosstalk identifier can be configured to operate at alocation remote from the receiver and the transmitters.

[0018] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

[0020]FIG. 1 is a block diagram of a subscriber line based communicationsystem having a plurality of twisted pair phone lines that extend froman optical network unit or central office to receivers in remote unitsand a third party data collection and crosstalk identification site, inaccordance with one embodiment of the present invention.

[0021]FIG. 2a is an illustration of near-end crosstalk interferencebetween a pair of communication lines.

[0022]FIG. 2b is an illustration of far-end crosstalk interferencebetween a pair of communication lines.

[0023]FIG. 3 is an illustration of a generic crosstalk model for areceiver in an xDSL system, in accordance with one embodiment of thepresent invention.

[0024]FIG. 4 is a typical baseband crosstalk diagram for an xDSL system,in accordance with one embodiment of the present invention.

[0025]FIG. 5 is a diagrammatic representation of the time stampmisalignment of a transmitted crosstalk signal and the signal of areceiver.

[0026]FIG. 6 is an example of a simulated cross-correlation for HDSLNEXT calculated using 1000 data samples and showing the absolutecross-correlation value as a function of the timing offset of atransmitted set of crosstalk data.

[0027]FIG. 7 is a graphical comparison of the crosstalk estimation erroras a function of the number of data samples used to identify thecrosstalk response, further compared to the combined noise and FEXTinterference in the received signal.

DETAILED DESCRIPTION OF THE INVENTION

[0028] Methods, devices and protocols appropriate for implementing asystem for identifying crosstalk in xDSL systems will now be describedin detail with reference to a few preferred embodiments thereof and asillustrated in the accompanying drawings. In the following description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art, that the present invention may be practicedwithout some or all of these specific details. In other instances, wellknown process steps and/or other features and aspects of the inventionas presented have not been described in detail in order to avoidunnecessarily obscuring the present invention.

[0029] Briefly, the present invention involves collecting data relatingto the transmission and reception of data on communication lines inwhich crosstalk arises. This collected data is then cross-correlated todetermine the timing differences between various lines' signals. Furtherprocessing of the data yields better estimates of the signals' timingdifferences and identification of crosstalk interference between thelines. The data processing procedures may be applied iteratively,removing successive levels of crosstalk to permit identification ofadditional crosstalkers, especially weaker crosstalkers such as FEXTsignals. Finally, the information obtained regarding the crosstalkinterference can be used to reduce or eliminate crosstalk on the lines,to assist in spectral management and assignments, and to assist lineoperators in maintaining, diagnosing and expanding services provided onthe lines.

[0030]FIG. 1 is a block diagram of an exemplary telecommunicationsnetwork 100 suitable for implementing the invention. Thetelecommunications network 100 includes a central office 102. Thecentral office 102 services a plurality of distribution posts to providedata transmission to and from the central office 102 to various remoteunits. In this exemplary embodiment, each of the distribution posts is aprocessing and distribution unit 104 (node). The processing anddistribution unit 104 is coupled to the central office 102 by a highspeed, multiplexed transmission line 106 that may take the form of afiber optic line. Typically, when the transmission line 106 is a fiberoptic line, the processing and distribution unit 104 is referred to asan optical network unit (ONU). The central office 102 also usuallyinteracts with and couples to other processing and distribution units(not shown) through high speed, multiplexed transmission lines 108 and110. In one embodiment, the processing and distribution unit 104includes at least one modem (central modem).

[0031] The processing and distribution unit 104 services a multiplicityof discrete subscriber lines 112-1 through 112-n. Each subscriber line112 typically services a single end user. The end user has a remote unitsuitable for communicating with the processing and distribution unit 104at very high data rates. More particularly, a remote unit 114 of a firstend user 116 is coupled to the processing and distribution unit 104 bythe subscriber line 112-1, and a remote unit 118 of a second end user120 is coupled to the processing and distribution unit 104 by thesubscriber line 112-n. The remote units 114 and 118 include a datacommunications system capable of transmitting data to and receiving datafrom the processing and distribution unit 104. In one embodiment, thedata communication systems are modems. The remote units 114 and 118 canbe incorporated within a variety of different devices, including forexample, a telephone, a television, a monitor, a computer, aconferencing unit, etc. Although FIG. 1 illustrates only a single remoteunit coupled to a respective subscriber line, it should be recognizedthat a plurality of remote units can be coupled to a single subscriberline. Also, as shown in FIG. 1, subscriber lines 132-1 through 132-n canbe connected directly to the central office 102 and be bundled in ashielded binder 142.

[0032] The subscriber lines 112 serviced by the processing anddistribution unit 104 are bundled in a shielded binder 122 as thesubscriber lines 112 leave the processing and distribution unit 104. Theshielding provided by the shielded binders 122 and 142 generally serveas good insulators against the emission (egress) and reception (ingress)of electromagnetic interference. However, the last segment of each ofthese subscriber lines, commonly referred to as a “drop” branches offfrom the shielded binder 122 and is coupled directly or indirectly tothe end user's remote units. The “drop” portion of the subscriber linebetween the respective remote unit and the shielded binder 122 isnormally an unshielded, twisted-pair wire. In most applications thelength of the drop is not more than about 30 meters.

[0033] Crosstalk interference occurs primarily within the shieldedbinder 122 where the subscriber lines 112 are tightly bundled, althoughcrosstalk can arise in other locations as well, including among lines indifferent binders. Hence, when data is transmitted on some of thesubscriber lines 112 while other subscriber lines are receiving data,the crosstalk inference induced becomes a substantial impairment toproper reception of data. There are two types of crosstalk interferencethat typically are of concern. Near-end crosstalk (NEXT), shown in FIG.2a, is interference that appears on a primary line 201 at the same endof the line as the source of the interference 203. Its level issubstantially independent of the length of the line and tends to be thepredominant type of crosstalk interference found in xDSL, systems.Far-end crosstalk (FEXT), shown in FIG. 2b, is interference that appearsat the end of a line 201 opposite or farthest from the end of the linecausing the interference 205.

Data Collection

[0034] The generic crosstalk model of an xDSL system for a givenreceiver is shown in FIG. 3. A series of data streams 310-0 through310-k are sent through a series of transmission lines 320-0 through320-k, respectively. For purposes of identifying crosstalk functions inthis disclosure, one line 320-0 is considered the primary channel h₀,while the remaining lines 320-1 through 320-k are crosstalk functions h₁through h_(k), respectively. The primary signal on channel ho, thecrosstalk interference and noise 330 are all combined and finallyreceived at a receiver 360.

[0035] It is much easier to identify crosstalk if all of the transmitteddata and the received data are known. In practice, as noted above,multiple parties (including perhaps competitors) may be using variouslines in bundles where crosstalk poses problems. Despite the competitivenature of the parties' relationship(s), it nevertheless appears to be inall participating parties' best interests to reduce crosstalkinterference as much as possible.

[0036] With line operators not wishing to share confidentialinformation, but having mutual interests in reducing and/or eliminatingcrosstalk, some level of objectively managed cooperation is preferred.Based on this fact, and as shown in FIG. 1, the preferred embodiment ofthe present invention uses an impartial third party site 150 to collectthe transmitted data and the received data during a specified timeperiod from all of the modems that are available for the couplingfunctions (crosstalk) identification. While the data collection site maybe operated by an impartial third party, the site may nevertheless belocated anywhere, including the central office or any other appropriatelocation. For example, the telephone company that operates the lines mayestablish an on-site facility in the central office to use the presentinvention in the maintenance and other line services provided for itsown use and the use of its customers.

[0037] The needed level of coordination can be achieved by establishinga standardized procedure in which each operator captures the data thatflow through the operator's modem(s) during a pre-defined time periodand sends the captured data to the third party. For example, in acentral office, service operators typically have their own DSL accessmultiplexers (DSLAMs) which are used to collect the transmitted andreceived data in each modem during a certain time period. Thesecollected data can be sent to the third party site via internet or someother means 160. At a “customer site,” each modem can store thetransmitted and received data packets and send them to the third partysite. Because line characteristics usually do not change very much,these data packets can be sent either offline when the modems are idleor via low-speed diagnostic channels currently used in all DSL modems.

Data Processing

[0038] The primary objective of the present invention is to identify oneor more crosstalk functions, given the known transmitted data, thereceived data and the statistics of the noise. The collected dataprovide much of this information, as will be appreciated by one of skillin the art. Processing techniques and apparatus of the present inventionpermit analysis and processing of the collected data and yield thedesired crosstalk function information.

[0039] As noted above, data are collected from modems within thecommunication system being evaluated. Typically, each modem time stamprelies on the central office clock. Unfortunately, these time stamps arenot accurate and the difference from modem to modem can be as large asseveral milliseconds. Moreover, different services can exist in the samebundle(s) of telephone lines and these services frequently havedifferent sampling rates. Therefore, a discrete (sampled) crosstalkfunction will vary with time if the receiver and the crosstalktransmitter belong to different services and have different symbolrates. However, if the transmitted data are resampled with the sameclock used in the receiver, the crosstalk function is stationary becauseit reflects the physical configuration of the lines.

Resampling for Different Services

[0040]FIG. 4 shows a typical baseband crosstalk diagram for xDSLsystems, where p(t), h(t) and h_(lp)(t) are the transmit filter 410, thecrosstalk response 420, and the receiver low pass filter 430respectively. The sampling rates for the primary transmitter and thereceiver are 1/T and 1/T′, respectively. The transmitted continuous timesignal is: $\begin{matrix}{{x_{c}(t)} = {\sum\limits_{l = 0}^{N - 1}{a_{l}{\delta \left( {t - {l\quad T} - \tau} \right)}*{p(t)}}}} & (1)\end{matrix}$

[0041] where T is the sampling period, a_(l) is the discrete datastream, N is the total number of data symbols to be transmitted, and τis the fractional delay in terms of the receiver clock. The receivedsignal before sampling is: $\begin{matrix}{{y_{c}(t)} = {{\sum\limits_{l = 0}^{N - 1}{a_{l}{\delta \left( {t - {l\quad T}} \right)}*\underset{\underset{q{(t)}}{}}{p\left( {t - \tau} \right)*{h(t)}*{h_{lp}(t)}}}} + {n(t)}}} & (2) \\{\quad {= {{\sum\limits_{l = 0}^{N - 1}{a_{l}{\delta \left( {t - {l\quad T}} \right)}*\frac{1}{T^{\prime}}\sin \quad {c\left( \frac{t}{T^{\prime}} \right)}*{q(t)}}} + {n(t)}}}} & (3) \\{\quad {= {{\frac{1}{T^{\prime}}\underset{\underset{x{(t)}}{}}{\sum\limits_{l = 0}^{N - 1}{a_{l}\sin \quad c\left( \frac{t - {l\quad T}}{T^{\prime}} \right)}}*{q(t)}} + {n(t)}}}} & (4)\end{matrix}$

[0042] where q(t) is the aggregated crosstalk function of concern andx(t) is the re-constructed transmitted signal. Eq. (3) above followsfrom the fact that y_(c)(t) is not changed by multiplying anotherlow-pass filter (1/T′) sinc(1/T′) if the receiver low pass filter h_(lp)is ideal. The bandwidth of the crosstalk function q(t) is determined bythe smaller one of the two filters p(t) and h_(lp)(t). In other words,the identifiable band of the crosstalk is transmitted by both the signalbandwidth and the receiver cut-off bandwidth. Sampling at a rate of 1/T′yields the discrete received signal:

y(m)=x(m)*q(m)+n(m)  (5)

[0043] where $\begin{matrix}{{x(m)} = {\sum\limits_{l = 0}^{N - 1}{a_{l}\sin \quad {{c\left( {m - \frac{l\quad T}{T^{\prime}}} \right)}.}}}} & (6)\end{matrix}$

[0044] Since both x(t) and q(t) have a bandwidth less than the Nyquistrate 1/(2T′), there is no aliasing after sampling. The resampling sincfunction is preferred, but not unique; many other functions (forexample, the raised cosine) can be used as alternative resamplingfunctions.

[0045] The resampled data x(m) are usually non-stationary and weaklycorrelated. Therefore any performance analysis should be used cautiouslyif it is based on the assumption that the transmitted data are white.Nevertheless, simulation results still suggest that the residual errorestimation is acceptable. These results are not so surprising becausethe data a_(l) before resampling are white and thus the resampled datashould contain all the necessary modes to excite all the frequencies ofinterest. If the transmitted signal is upsampled, the covariance matrixZ*Z (where*represents conjugate and transpose; the Z matrix is discussedin more detail below) made of x(m) is close to singular and theleast-squares estimator can not be applied directly. One solution is toexpand the bandwidth of the signal x(m) to the bandwidth of the receiverfilter during the resampling process. This expansion of the bandwidthwill give the covariance matrix a good condition number and cause anegligible effect in the estimation error.

First Timing Offset Estimation

[0046] Even though the data from various services have been resampled tocorrect for sampling rate differences, as mentioned above, differentmodems from which data are collected may have different time stamps.Consequently, pre-defined time spans defining the time period duringwhich data are collected from different modems might not be strictlyaligned together, as shown in FIG. 5. Even though they have the samenumber of data samples L+1 (that is, are the same length), a transmittedsignal x(m) 510 may lag or lead a received signal y(m) 520 by a timingdifference or offset, d_(i) 530. Therefore, in the present invention,the collected data are initially evaluated to determine whether timingoffsets exist between the received signal and the transmitted crosstalksignals' data. If such timing offsets are present, they can beestimated.

[0047] To start the analysis of the respective timing offsets betweenthe received signal and the transmitted signals, and without a loss ofgenerality, all transmitters are assumed to have non-negative timingdifferences (d_(i)≧0) with respect to the time stamp of the designatedreceiver. Using the data available for analysis, the received signal canbe characterized as: $\begin{matrix}{{y(m)} = {{\sum\limits_{i = 0}^{K}{{h_{i}(m)}*{x_{i}\left( {m + d_{i}} \right)}}} + {n(m)}}} & (7)\end{matrix}$

[0048] where

[0049] x_(i): the transmitted data

[0050] h_(i): the channel response (i=0) or the crosstalk function (i>0)

[0051] d_(i): the timing difference

[0052] n: white Gaussian noise

[0053] K: the number of crosstalkers.

[0054] In general, the channel response h₀(m) is known and d₀ equalszero because the transmitter and the receiver at both ends of the samechannel are synchronized. As a result, the originally transmitted signalcomponent can be subtracted from the received signal. Therefore, theremaining crosstalkers can be considered.

[0055] Since the timing differences d_(i) could be several thousandsymbols, it is computationally prohibitive to apply the classicalleast-squares method directly to Eq. (7). A cross-correlation is firstused to estimate the timing differences d_(i) of the dominantcrosstalkers. Where a primary signal contains contamination from asecondary, known signal (as in the case of crosstalk in communicationslines), a cross-correlation of the two signals will yield informationregarding the relative “spacing” of the signals. That is, approximatetiming offsets of primary and crosstalk signals can be determined bylooking for indications of a crosstalker's interfering signal in thereceived signal and evaluating various offsets of the crosstalker'ssignal in the received signal. An estimate of a timing offset can thusbe found which yields a relatively high correlation between the receivedsignal and the crosstalk generating signal. By getting a “ballpark”estimate of the timing offsets, a classical least squares approach canlater be applied with reliability.

[0056] Once the timing differences of the significant crosstalkers areapproximated, the cross talk functions are identified by a least-squaresestimator and the crosstalkers are subtracted from the received signal.The subtraction of the dominant crosstalkers makes it easier to estimatethe timing difference for the smaller crosstalkers. This successivecancellation process may be repeated until all interested crosstalkfunctions are identified.

Timing Differences Estimation Using Cross-Correlations

[0057] Cross-correlation is used in the present invention in a new wayfor xDSL systems. Specifically, there are many crosstalks, which haveunknown responses h_(i) with many taps. The exact delay estimation isnot required in this step. Instead, a relatively coarse first estimateof each timing difference d_(i) is obtained. As will be seen below, suchan estimation permits identification (and removal) of significantcrosstalkers, especially in connection with subsequently seeking out andidentifying smaller crosstalkers in the received signal.

[0058] The cross-correlation between the received signal y(m) and thecrosstalk transmission signals is defined as:

R _(yx) _(i) (l)=E{y(m)x _(i)*(m+l)}.  (8)

[0059] For purposes of this analysis, the transmitted data fromdifferent users are assumed to be independent and have zero mean, i.e.,E(x_(i)x_(j))=0 for i≠j, and E(x_(i))=0. The noise is uncorrelated withall users' data. The number of taps for h_(i) is assumed to be v_(i)+1.Then by substitution of y(m), Eq. (8) can be rewritten as:$\begin{matrix}{{R_{y\quad x_{i}}(l)} = {\sum\limits_{t = 0}^{v_{i}}{{h_{i}(t)}{R_{x_{i}}\left( {l - d_{i} + t} \right)}}}} & (9)\end{matrix}$

[0060] where R_(x)is the autocorrelation function of x_(i). In the idealcase where the transmitted data x_(i) are uncorrelated and have anaverage energy of ε_(i) (that is, where R_(x)(k) =ε_(i)δ(k)):

R _(yx) _(i) (l)=ε_(i) h _(i)(d _(i) −l).  (10)

[0061] Because the crosstalk function is a causal finite impulseresponse (FIR), the timing delay d_(i) can be estimated by the maximuml_(d) such that |R_(yx) _(i) (l_(d))|>0. Unfortunately, theautocorrelation of the transmitted data is rarely a delta function inxDSL systems and can only be obtained approximately by averaging over alarge number of data samples. Therefore, it is difficult to identify thecrosstalk function directly from Eq. (9). Nevertheless, it is possibleto use this equation to roughly estimate the timing difference d_(i) bysearching for the peak of |R_(yx) _(i) (l_(d))|, that is:$\begin{matrix}{{\overset{\sim}{d}}_{i} = {\arg \quad {\max\limits_{l}{{{R_{{yx}_{i}}(l)}}.}}}} & (11)\end{matrix}$

[0062] The applicability of this method is strongly justified when oneobserves that the transmitted data are very loosely correlated (theauto-correlation function R_(x) _(i) (k) is almost equal to zero if k islarger than several symbols), and that hi(t) is an FIR filter and has anarrow peak in the time domain.

[0063] The cross-correlation R_(yx) _(i) is approximated by averagingover many data samples: $\begin{matrix}{{{\overset{\sim}{R}}_{y\quad x_{i}}(l)} = {\frac{1}{N}{\sum\limits_{m = 1}^{N}{{y(m)}{x_{i}^{*}\left( {m + l} \right)}}}}} & (12)\end{matrix}$

[0064] where N is the total number of the data symbols used foraveraging. As will be appreciated by those skilled in the art, thegreater the number of data samples N that are used, the more accuratethe approximation of the cross-correlation. However, the computationalcomplexity of this method is approximately O(Nd_(i)) for user i.Therefore, from a practical standpoint, the number of data symbols Nshould be carefully selected to achieve a relatively good balancebetween the computational complexity and the accuracy of the estimation.The rough estimation of N is discussed below.

[0065] The mean of the cross-correlation {tilde over (R)}_(yx) _(i) (l)is E({tilde over (R)}_(yx) _(i) (l)=R_(yx) _(i) (l). The variance of thecross-correlation R_(yx) _(i) is: $\begin{matrix}{\sigma_{R}^{2} = {{var}\left( {{\overset{\sim}{R}}_{y\quad x_{i}}(l)} \right)}} & (13) \\{\quad {\approx {\frac{ɛ_{i}}{N}\left( {{\sum\limits_{j = 1}^{K}{ɛ_{j}{h_{j}}^{2}}} + \sigma_{n}^{2}} \right)}}} & (14)\end{matrix}$

[0066] where ∥h_(j)∥ is the norm of the crosstalk function h_(j)(t).This above approximation is surprisingly simple in which user jcontributes ε_(j)∥h_(j)∥² to the variance. To detect the peak value ofthe cross-correlation |{tilde over (R)}_(yx) _(i) (l)| reliably, theratio of the peak to the standard deviation σ_(R) can be set to a largevalue, for example: $\begin{matrix}{{\frac{\max \left( \left| {{\overset{\sim}{R}}_{y\quad x_{i}}(l)} \right| \right)}{\sigma_{R}} > a} = 10} & (15)\end{matrix}$

[0067] This is equivalent to: $\begin{matrix}{N > \frac{a^{2}{ɛ_{i}\left( {{\sum\limits_{j = 1}^{K}{ɛ_{j}{h_{j}}^{2}}} + \sigma_{n}^{2}} \right)}}{\left( {\max \left( {{\overset{\sim}{R}}_{y\quad x_{i}}(l)} \right)} \right)^{2}}} & (16)\end{matrix}$

[0068] If all users are assumed to have the same energy (that is,ε_(i)=e_(j)) and the transmitted data are temporally uncorrelated, theinequality can be further simplified to $\begin{matrix}{N > \frac{a^{2}\left( {{\sum\limits_{j = 1}^{K}{h_{j}}^{2}} + {\sigma_{n}^{2}/ɛ_{i}}} \right)}{\left( {\max \left( \left| h_{i} \right| \right)} \right)^{2}}} & (17)\end{matrix}$

[0069] which provides us a good guidance to select a reasonable numberfor N. In typical xDSL systems, the background noise n is very small(−140 dBm) so that $\frac{\sigma_{n}^{2}}{ɛ_{i}}$

[0070] in Eq. (17) can be ignored in estimating N. Two particularexemplary cases are presented here as illustrative.

EXAMPLE 1.

[0071] Suppose there are 10 crosstalkers (K=10) that have the sameorders of magnitude, that is, ∥h_(j)∥≈∥h_(i)∥≈max(∥h_(i)∥) for all j.Then, reducing Eq. 8 accordingly (with a=10, as noted above), N≈a²K=1000provides a good estimation (that is, 1000 samples should yield anaccurate estimate of the timing difference).

EXAMPLE 2.

[0072] Suppose that crosstalker 2 is dominant, and is 20 dB above theother crosstalkers, that is, ∥h₂∥≈10∥h_(i)∥,i≠2. Then for crosstalker 2,N≈a²=100. However, to estimate the timing offset of other crosstalkers,N≈100a²=10,000. If there are not enough data available, the dominantcrosstalkers need to be canceled first and the timing offset estimationfor the others is performed afterwards. In some cases, the smallercrosstalkers can be ignored; they will not substantially affect thereceived signal. Once the large crosstalker in this example is removed,the other crosstalk signals can be addressed.

Crosstalk Function Estimations

[0073] After estimating the coarse timing differences d_(i) between theinput(s) from any considered crosstalk signal(s) and the receivedsignal, the crosstalk functions and more accurate timing differences canbe obtained jointly by a least-squares estimator. In some cases, if somecrosstalkers are too small, then the timing differences found throughcross-correlation may not be accurate enough for the least-squaresestimator. Depending on the objective, those small crosstalkers caneither be ignored or they can be identified after finding the strongcrosstalkers first and subtracting those strong crosstalkers from thereceived signal. Once the strong crosstalkers are removed, the methodsand apparatus of the present invention can be applied in an iterativefashion to identify (and remove if necessary or desirable) successivelyweaker crosstalk interference signals.

[0074] The error Δ_(i) in the timing difference estimation determinedusing the cross-correlation technique above can be expressed asΔ_(i)=d_(i)−{tilde over (d)}_(i) and the shifted input asz_(i)=x_(i)(m+{tilde over (d)}_(i)). Then the crosstalk network model ofEq. 7 can be rewritten (using the same notations) as: $\begin{matrix}{{y(m)} = {{\sum\limits_{i = 1}^{K}{{h_{i}(m)}*{z_{i}\left( {m + \Delta_{i}} \right)}}} + {n(m)}}} & (18)\end{matrix}$

[0075] In the ideal case where the transmitted signals are uncorrelated(that is, where R_(x) _(i) (m)=ε_(i)δ(m)), Δ_(i) is the tap number wherethe absolute crosstalk response |h_(i) (Δ_(i)) |has a peak value. Inpractice, typically, the transmitted data are weakly correlated. As aresult, d_(i) should be in the vicinity of the tap where the absolutecrosstalk response has its maximum. That is, mathematically, Δ_(i)≈argmax m (|h_(i) (m)|) and 0≦Δ_(i)≦v_(i).

[0076] Note that the exact length of the crosstalk response v_(i)+1 isunknown. Fortunately, in xDSL systems, the upper bound of the length isempirically available, which can be used to approximate v_(i). Jointdetection of the crosstalk response and the delay is based on thefollowing:

[0077] 1) the length of the crosstalk response is assumed to be 2v_(i)+1in order to include the effect of the timing offset Δ_(i);

[0078] 2) a least-squares estimator is used to estimate the crosstalkresponse of length 2v_(i)+1; and

[0079] 3) those taps whose coefficients are almost equal to zero arefound in the head and the tail of the considered crosstalk response oflength 2v_(i)+1, which can be used to find the timing offset Δ_(i).

[0080] By truncating the taps that are almost zero, the crosstalkresponse is obtained. The matrix representation of Eq. (18) can bewritten as: $\begin{matrix}{y = \begin{matrix}\left\lbrack Z_{1} \right. & Z_{2} & \ldots & {{\left. Z_{K} \right\rbrack \cdot \begin{bmatrix}h_{1} \\h_{2} \\\vdots \\h_{K}\end{bmatrix}} + n}\end{matrix}} & (19)\end{matrix}$

 =Z·h+n  (20)

[0081] where y is the received signal vector, Z_(i)εC^(Nx(v) ^(_(i)) ⁺¹⁾is the transmitted Toeplitz data matrix, and h_(i) is the crosstalkresponse vector, (i=1, . . . , K) as shown more specifically as follows:

y=[y(N−1) y(N−2) . . . y(0)]^(T)  (21)

[0082] $\begin{matrix}{Z_{i} = \begin{bmatrix}{z_{i}\left( {N - 1 - v_{i} + \Delta_{i}} \right)} & \ldots & {z_{i}\left( {N - 1 + \Delta_{i}} \right)} \\\vdots & ⋰ & \vdots \\{z_{i}\left( {{- v_{i}} + \Delta_{i}} \right)} & \ldots & {z_{i}\left( \Delta_{i} \right)}\end{bmatrix}} & (22)\end{matrix}$

 h _(i) =[h _(i)(v _(i)) h _(i)(v _(i)−1) . . . h _(i)(0)]^(T)  (23)

[0083] where N is the number of data samples and ¹ represents transpose.Since Δ_(i)ε[0, v_(i)], we can extend each row of the data matrix Z_(i)and the crosstalk response vector h_(i) in both directions, as follows:

h _(i) =[h _(i)(v _(i)+Δ_(i)) . . . h _(i)(v _(i)) . . . h _(i)(0) . . .h _(i)(Δ_(i) −v _(i))]^(T)  (24)

[0084] $\begin{matrix}{Z_{i} = \begin{bmatrix}{z_{i}\left( {N - 1 - v_{i}} \right)} & \ldots & {z_{i}\left( {N - 1 + v_{i}} \right)} \\\vdots & ⋰ & \vdots \\{z_{i}\left( {- v_{i}} \right)} & \ldots & {z_{i}\left( v_{i} \right)}\end{bmatrix}} & (25)\end{matrix}$

[0085] where Z_(i) εC^(Nx(2v) ^(_(i)) ⁺¹⁾ and h_(i) εC^((2v) ^(_(i))^(+1)x1). The same notations are used in Eqs. (21) through (25) to avoidnotation explosion. Note that h_(i) are padded with Δ_(i) zeros on thetop and v_(i)−Δ_(j) zeros in the bottom. From Eq. (20), the unbiasedestimation of h is as follows:

ĥ=( Z*Z)⁻¹ Z*y  (13)

[0086] where * represents conjugate and transpose. By truncating thosetaps corresponding to relatively small values in both sides of thevector ĥ, we obtain the crosstalk response and the timing offset Δ_(i).Those of ordinary skill in the art will be aware of fast algorithms forcalculating Eq. (26) utilizing the Toeplitz structure of the datamatrix. Specific examples can be found in the following:

[0087] J. M. Cioffi and T. Kailath, “Fast, Recursive-Least-SquaresTransversal Filters for Adaptive Filtering”, IEEE Trans. Acoust.,Speech, Signal Proc., vol. ASSP-32, April 1984, p.304-336; and

[0088] J. M. Cioffi, “The Block-Processing FTF Adaptive Algorithm”, IEEETrans. Acoust., Speech, Signal Proc., vol. ASSP-34, February 1986,p.77-90.

[0089] Selection of the number of data symbols N in the presentinvention can yield good estimation errors. Generally, it has been foundthat the estimation error level can be kept at about the same level asthe noise when the number of data symbols is at least twice the numberof estimated parameters M. Doubling the number of data symbols againwould reduce the estimation error by another 3 dB. For the complex inputdata, the equations are slightly different, but the rule still applies.

Simulation Results

[0090] Simulation results of the crosstalk identification of the presentinvention in the upstream direction (from the subscriber to the centraloffice) have produced some very favorable results. The receiver isassumed to be an ADSL modem. The number and the type of the crosstalksare assumed in the following to reflect the typical crosstalkenvironment:

[0091] 4 Basic Rate ISDNs (BRIs)

[0092] 4 HDSLs

[0093] 5 ADSLs

[0094] All twisted pairs are assumed to be 26-gauge (0.4 mm) and9000-feet (2744 m) long. The dominant crosstalks consist of NEXTs fromBRIs and HDSLs. There is no NEXT from ADSL because most of the deployedADSL modems use a frequency-division duplexing scheme. The smallercrosstalks consist of FEXTs from ADSLs, BRIs, and HDSLs. Severalcharacteristics of xDSLs from Annex B of the ADSL standard (ANSIStandard T1.413 for ADSL) also were used.

[0095] The transmit filters (including pulse shaping filters) fromdifferent types of DSLs can also be found in the ADSL standard or thecorresponding standards. The models for NEXT and FEXT are commonlyknown.

[0096] In the simulation, each crosstalk signal was transmitted with arandom timing offset with respect to the ADSL receiver. This integeroffset was uniformly distributed. The fractional delay was absorbed intothe crosstalk function as described above in Eq. (4). The NEXTs wereidentified first. The cross-correlation technique was used to grosslyestimate these timing offsets. As expected, and as shown in an examplein FIG. 6 the peak value 610 was much higher than the rest of values(for example, values 620) and gave a good first estimate of the timingoffset.

[0097] With the first estimate of the timing offset, a least-squaresestimator was used to identify the crosstalk responses. The results ofthis process were consistent with the results predicted above. As seenin FIG. 7, when the number of data samples N was greater than twice thenumber of estimated parameters, the crosstalk function estimation error(line 710) generally was comparable to the noise (line 720), which isgenerally quite low. In fact, N=2M+1 was a critical turning point (shownapproximately at point 730), where M is the number of estimatedparameters. In the simulation, M was:$M = {{\sum\limits_{i = 1}^{8}\left( {v_{i} + 1} \right)} = 248}$

[0098] When N>2M, doubling the number of the data symbols reduced theerror 3 dB. However, the error grew rapidly when the amount of data wasless than 2M.

[0099] It also may be useful to identify smaller crosstalkers, such asFEXTs. Since they are much smaller than NEXTs, the NEXTs are firstidentified and subtracted from the received signal. The above steps ofcross-correlation and least-squares estimation can be applied again inone or more successive applications to the received signal (now lackingthe stronger NEXT crosstalkers) to find the FEXT functions (and anyother weaker crosstalk signals). The simulation results were similar.The case where the real measured NEXTs and FEXTs data are used also wassimulated. The results again were consistent with the analysis.

[0100]FIG. 8 illustrates briefly one embodiment of the methodology ofthe present invention. Initially, at 802, data y(m) is collected fromthe receiver and data transmitted data x_(i) (m) is collected from thetransmitters. At 804, any of the data that were originally sampled atrates different then the receiver's rate are resampled. At 806 theprimary transmitted data x₀ (m) may be subtracted if desired from thereceived data y(m) to yield interference data which includes crosstalkdata. At 808, if timing offsets exist between the received data and thecrosstalk data, a cross-correlation of the received data with each suchcrosstalk signal is constructed at 808. At 810 a peak is determined ineach cross-correlation to determine the first estimate d_(i) of thetiming offset between the received signal and the considered crosstalksignal. Using d_(i) and the transmitted crosstalk data, at 812, eachcrosstalk response h_(i) and a second estimate of each timing offsetΔ_(i) are estimated using a least-squares estimation. At 814, a decisionis made as to whether there are more crosstalk signals to identify. If“yes”, at 816, then the procedure just described beginning at 808 can berepeated (successively, if appropriate). For example, if strongcrosstalk signals are going to be removed first and weaker signals foundand identified later, then the process of FIG. 8 can be applied to thestrong crosstalkers, those strong crosstalkers removed, and weakercrosstalkers found thereafter using this process. If “no” furthercrosstalk signals are to be identified, at 818, the identified crosstalkresponses can be used for line services as described herein. Of course,variations on this embodiment will be apparent to those of ordinaryskill in the art, depending on the particular goals of the analysis andcrosstalk identification.

[0101] As noted above, it is contemplated that the identification ofcrosstalk according to one or more embodiments of the present inventioncould be used in various services relating to DSI, systems. One exampleof such services is dynamic spectrum management. Unbundling architectureat the remote terminal, the “central office side” of a DSL line, isstill largely undecided. Spectrum management and DSL standardization todate have investigated only the situations where service providersenergize the physical transmission lines with independently generatedsignals. Spectra for the different types of DSL signals has beenspecified for worst-case crosstalking situations with the goal ofminimum service disruption among the various DSL signals. Necessarily,such static worst-case spectrum management reduces achievable data ratesand symmetries with DSL.

[0102] The results of application of the present invention could be usedin connection with other features relating to improving DSL technologythrough dynamic spectra management. For example, entities controllingspectra could be informed of identified crosstalk functions and otherresults of the application of the present invention. Moreover, thatinformation could be used to adjust the spectra of one or more DSL linesto improve performance. Finally, systems could be developed forcoordinating line spectra and line signals (for example, vectoring) toimprove DSL performance. More specifically, crosstalk functionsaffecting a number of transmitted signals in transmission lines areidentified using the present invention. The transmitted signals can thenbe synchronized at the their respective transmitters. Additionally, thetransmitted signals can also be coordinated to mitigate or cancel themutual crosstalk that affects those signals. Similarly, receivers can becoordinated to mitigate their signals. Again, crosstalk functionsaffecting the signals received by the receivers can be identified usingthe present invention. After the received signals are collected fromtheir respective receivers, the mutual crosstalk affecting the signalscan be mitigated or canceled using digital signal processing.

[0103] The results of application of the present invention could be usedin connection with other features relating to improving DSL technologythrough dynamic spectra management. For example, entities controllingspectra could be informed of identified crosstalk functions and otherresults of the application of the present invention. Moreover, thatinformation could be used to adjust the spectra of one or more DSL linesto improve performance. Finally, systems could be developed forcoordinating line spectra and line in at the signal level (for example,vectoring) to improve DSL performance. More specifically, crosstalkfunctions affecting a number of transmitted signals in transmissionlines are identified using the present invention. The transmittedsignals can then be synchronized at the their respective transmitters.Additionally, the transmitted signals can also be coordinated tomitigate or cancel the mutual crosstalk that affects those signals.Similarly, receivers can be coordinated to mitigate their signals.Again, crosstalk functions affecting the signals received by thereceivers can be identified using the present invention. After thereceived signals are collected from their respective receivers, themutual crosstalk affecting the signals can be mitigated or canceledusing digital signal processing.

[0104] It is intended that the following appended claims be interpretedas including all such alterations, permutations, and equivalents as fallwithin the true spirit and scope of the present invention.

What is claimed is:
 1. A method of identifying crosstalk in a receivedsignal, the method comprising: collecting received data corresponding tothe received signal from a receiver; collecting primary data from aprimary transmitter; collecting crosstalk data from a crosstalktransmitter; identifying a crosstalk function corresponding to thecrosstalk data.
 2. The method of claim 1 further comprising determininga first estimate of a timing offset between the received data and thecrosstalk data.
 3. The method of claim 2 wherein determining the firstestimate of the timing offset comprises cross-correlating the receiveddata and the crosstalk data if the timing offset is other than zero. 4.The method of claim 1 wherein identifying the crosstalk functioncomprises performing an estimation from the group comprising a standardleast-squares estimation and a weighted least-squares estimation.
 5. Themethod of claim 4 wherein performing an estimation includes jointlydetermining an estimate of the timing offset between the received dataand the crosstalk data and identifying a crosstalk functioncorresponding to the crosstalk data.
 6. The method of claim 1 furthercomprising: collecting a plurality of sets of crosstalk data from aplurality of crosstalk transmitters, including a first set of crosstalkdata from a first crosstalk transmitter; identifying a crosstalkfunction corresponding to the first set of crosstalk data.
 7. The methodof claim 6 further comprising determining a first estimate of a timingoffset between the received data and the first set of crosstalk data. 8.The method of claim 1 further comprising: collecting a plurality of setsof crosstalk data from a plurality of crosstalk transmitters, theplurality of sets of crosstalk data comprising a set of strong crosstalksignal data corresponding to a strong crosstalk signal and a set of weakcrosstalk signal data corresponding to a weak crosstalk signal;determining whether a timing offset exists between the received data andthe set of strong crosstalk signal data; generating a first estimate ofany determined timing offset between the received data and the set ofstrong crosstalk signal data; identifying a strong crosstalk functioncorresponding to the strong crosstalk signal data; subtracting thestrong crosstalk function from the received signal to generate amodified received signal; determining whether a timing offset existsbetween the received data and the set of weak crosstalk signal data;generating a first estimate of any determined timing offset between themodified received signal and the weak crosstalk signal data; andidentifying a weak crosstalk function corresponding to the weakcrosstalk signal data.
 9. The method of claim 2 further comprisingsubtracting the primary data from the received data prior to determiningthe first estimate of the timing offset between the received data andthe crosstalk data.
 10. The method of claim 1 further comprisingperforming multiuser detection using the identified crosstalk function.11. The method of claim 1 further comprising provisioning communicationlines in a DSL system in which the identified crosstalk function isidentified.
 12. The method of claim 1 further comprising performing DSLsystem diagnosis services for a DSL system in which the identifiedcrosstalk function is identified.
 13. The method of claim 1 furthercomprising providing DSL system maintenance services for a DSL system inwhich the identified crosstalk function is identified.
 14. The method ofclaim 1 further comprising performing spectral management services for aDSL system in which the identified crosstalk function is identified. 15.A system for identifying crosstalk comprising: a first transmitterconfigured to transmit a first signal; a second transmitter configuredto transmit a second signal; a receiver configured to receive a combinedsignal, the combined signal comprising the first signal and crosstalkinterference from the second signal; and a processor comprising: a datacollector in communication with the first transmitter, the secondtransmitter and the receiver, the collector configured to collect: afirst signal data set corresponding to the first signal; a second signaldata set corresponding to the second signal; and a combined signal dataset corresponding to the combined signal; a crosstalk identifierconnected to the data collector comprising: a crosstalk responseestimator configured to estimate the crosstalk interference present inthe combined signal.
 16. The system of claim 15 wherein the crosstalkidentifier further comprises a first timing offset estimator configuredto calculate a first estimate of a timing offset between the combinedsignal and the second signal.
 17. The system of claim 16 wherein thecrosstalk response estimator is configured to calculate a secondestimate of the timing offset.
 18. The system of claim 16 wherein thefirst timing offset estimator comprises a cross-correlator configured toperform a cross-correlation of the combined signal and the second signalto provide the first timing offset.
 19. The system of claim 15 whereinthe crosstalk identifier comprises a least-squares estimator.
 20. Thesystem of claim 15 wherein the first transmitter and the receiver arepart of a DSL communication system.
 21. The system of claim 15 whereinthe processor is located at a location remote from the first and secondtransmitters and the receiver.
 22. The system of claim 15 wherein thefirst transmitter, the second transmitter and the receiver are modems.23. The system of claim 15 wherein the processor further comprises adata conditioner, connected to the data collector, configured toresample collected data.
 24. The system of claim 15 wherein thecrosstalk identifier further comprises a data subtractor configured tosubtract the first signal from the combined signal to generate aninterference signal.
 25. A crosstalk identifier comprising: a collectorconfigured to collect data from a primary signal transmitter, from acrosstalk signal transmitter and from a receiver; a crosstalk estimatorin communication with the collector, the crosstalk estimator configuredto determine a first estimate of a crosstalk response in the data fromthe receiver.
 26. The identifier of claim 25 further comprising a timingestimator connected to the collector, the timing estimator configured todetermine a first estimate of a timing offset between the data from thereceiver and the data from the crosstalk signal transmitter.
 27. Theidentifier of claim 26 wherein the crosstalk estimator also isconfigured to determine a second estimate of the timing offset betweenthe data from the receiver and the data from the crosstalk signaltransmitter.
 28. The identifier of claim 26 wherein the timing estimatorcomprises a cross-correlator configured to provide a cross-correlationof the data from the receiver and the data from the crosstalk signaltransmitter to determine the first estimate of the timing offset. 29.The identifier of claim 25 wherein the crosstalk estimator comprises aleast-squares estimator configured to determine the first estimate ofthe crosstalk response.
 30. The identifier of claim 27 wherein thecrosstalk estimator uses a least-squares estimator to determine a secondestimate of the timing offset.
 31. The identifier of claim 25 whereinthe identifier is configured to be used at a third party site remotefrom the transmitters and the receiver.
 32. A method for identifyingcrosstalk in a received signal caused by interference from a crosstalksignal, the method comprising: collecting received data from a receiverthat has received the received signal during a specified time period;collecting primary data transmitted as a primary signal during thespecified time period; collecting crosstalk data transmitted as a firstcrosstalk signal during the specified time period; subtracting theprimary data from the received data to generate interference data;determining a first estimate of a timing offset between the receivedsignal and the first crosstalk signal, comprising cross-correlating theinterference data and the crosstalk data; identifying a crosstalkfunction corresponding to the crosstalk signal, comprising performing aleast-squares estimation to identify the crosstalk function and thecrosstalk signal using the interference data and the first estimate ofthe timing offset.
 33. A method of dynamically managing spectra in a DSLsystem, comprising: identifying crosstalk functions and characteristicsin the DSL system; transferring information concerning the identifiedcrosstalk functions and characteristics to an entity controlling spectrain the DSL system; controlling line spectra in modems in the DSL system.34. The method of claim 33 wherein the step of controlling line spectrain modems in the DSL system comprises: adjusting spectra in the DSLsystem to reduce crosstalk interference; coordinating the use of spectrain the DSL system to reduce crosstalk interference.
 35. The method ofclaim 33 wherein the step of identifying crosstalk functions andcharacteristics in the DSL system includes the step of identifyingcrosstalk in a received signal, identifying crosstalk in a receivedsignal comprising: collecting received data corresponding to thereceived signal from a receiver; collecting primary data from a primarytransmitter; collecting crosstalk data from a crosstalk transmitter;identifying a crosstalk function corresponding to the crosstalk data.36. A method of transmitter coordination in the signal level comprising:identifying crosstalk functions affecting a plurality of transmittedsignals on a plurality of transmission lines; synchronizing theplurality of transmitted signals; and coordinating the plurality oftransmitted signals to reduce crosstalk.
 37. A method of receivercoordination in the signal level comprising: identifying crosstalkfunctions affecting a plurality of signals on a plurality oftransmission lines; synchronizing the plurality of signal; and reducingcrosstalk among the plurality of signals using digital signalprocessing.